This invention relates to sputtering target materials of perovskite type composite oxide represented by the compositional formula Ba.sub.x Sr.sub.1-x TiO.sub.3-y, for use in depositing highly dielectric capacitor thin films of MOS-ULSI by sputtering process. This invention relates, more particularly, to sputtering target materials capable of remarkably decreasing the leakage current in the deposited Ba.sub.4 Sr.sub.1-x TiO.sub.3 thin film or to sputtering target materials capable of forming thin films with a minimum of particle defect and also with enhanced mechanical strength.
Research has in recent years been made on the use of high-dielectric-constant thin films of SrTiO.sub.3 and Ba.sub.x Sr.sub.1-x TiO.sub.3 that are obtained by substituting part of the Sr site of SrTiO.sub.3 with Ba for the capacitors of semiconductor memories. As a method of depositing such highly dielectric thin films, sputtering is commonly used. In order to impart desirable dielectric properties to the thin films formed by sputtering, a heat treatment to enhance their crystallinity is necessary. In that case, it is known that if the thin films contain alkali metal impurities such as Na and K, crystal growth will force these impurities to come out along the grain boundaries with a consequent increase of leakage current through these impurities. It is also known that U, Th, and other radioactive elements, if present as impurities, will emit alpha rays which can induce software error.
For instance, Japanese Patent Application Kokai No. 7-70747 clarifies that in a target material of a perovskite type titanate compound, what is responsible for leakage current is alkali metal that contributes largely to the migration of charges and teaches that the alkali metal content should be kept 1 ppm or less. It also proposes that the total amount of the radioactive elements such as U and Th that can cause software error by their alpha rays be limited to 10 ppb or less.
Patent Application Kokai No. 7-173621 discloses that on a sintered target material for sputtering composed of Ba.sub.x Sr.sub.1-x TiO.sub.3-y an upper limit of 10 ppm is put to the alkali metal impurity content so that high speed film deposition can be realized.
Patent Application Kokai No. 7-3444 suggests that in order to improve the resistance to cracking due to thermal strain of sintered target materials for sputtering composed of (Ba, Sr)O.TiO.sub.2, the total content of inevitable impurities such as Na and Ca, and also Al, Si, and Fe should be 90 ppm or less
Other problems associated with SrTiO.sub.3 and Ba.sub.x Sr.sub.1-x TiO.sub.3 that is obtained by substituting part of the Sr site of SrTiO.sub.3 with Ba are that since these materials tend to give rise to abnormal grain growth during sintering, they can hardly provide sputtering targets of dense structure. Sintering them at higher temperatures for obtaining denser structure will encourage the abnormal grain growth until sintered structures with grain diameters as large as 20 .mu.m or upwards result. When such a sintered body is used as a sputtering target, a large number of defects known as particles (clusters of minute particulates released from the target) develop on the deposited thin film, resulting in a serious decrease in the yield. Moreover, when sintered bodies are machined to target dimensions, those of low density or large grain size have such low mechanical strength that they present problems of low yield, contamination with impurities, etc.